MOSFET and IGBT output power devices are commonly integrated on the same silicon substrate with current sensors for producing a voltage signal representative of the current absorbed by the load driven by the power device that is to be provided to a control circuit. These sensors are essentially in the form of a sensing transistor that is a scaled replica of the power MOSFET or IGBT, and are biased with the same voltages.
Typically, the sensing transistor is a portion of the cellular array structure of a corresponding integrated power device. The current flowing in the sensing transistor is substantially proportional to the current flowing through the power device. By connecting in series with the sensing transistor one or more resistors, as depicted in FIG. 1, a generated voltage drop, detectable by a control circuit, is proportional to the current that the power device delivers to a load. This technique is preferably used since the alternative of connecting a sense resistor in series with the power device would generate an excessive voltage drop that may alter the bias conditions of the power transistor.
However, there is the drawback that the integrated sensing resistor connected in series to the scaled sensing transistor occupies a non-negligible silicon area. Moreover, its value needs to be determined with great precision to avoid unacceptable errors in the feedback regulation of the current flowing through the power transistor.
Typically, the sensing resistance is determined with trimming techniques. An array of identical highly precise resistors is integrated on the silicon substrate, and connected in parallel through fuses, as shown in the electrical scheme and in the layout view of FIGS. 2a and 2b. The desired value of the sensing resistance is defined by the user by burning certain fuses and leaving intact the others in order to adjust it to the sensitivity of the current measuring circuit. This technique implies a corresponding silicon area consumption because in general not all resistors are utilized in the electrical circuit. Some of them remain isolated after having burned the corresponding fuses.
Another drawback is that the sensing resistor modifies the biasing of the sensing transistor. To make an accurate sensing of the current absorbed by the load, the portion of the power transistor used as a sensing transistor and connected in series with the sensing resistor needs to have a voltage-current characteristic that is as similar as possible to that of the whole integrated power device, in any functioning condition. When such a condition is not satisfied, there may be errors in sensing the current, and as a consequence, an imprecise feedback control of the current flowing through the device may result. Usually, this operation may be critical and it is based on the user's experience. It is verified whether or not (and how much) the functioning of this sensing portion of the integrated power device structure is representative of the functioning of the whole device.